A boy, a girl and a dog are standing together on a long, straight road. Simultaneously, they all start walking in the same direction: The boy at 4 mph, the girl at 3 mph, and the dog trots back and forth between them at 10 mph. Assume all reversals of direction instantaneous.

Well if the boy and girl start walking, with the boy taking wider and faster steps than the girl, until as time goes by, the distance between boy and girl get further. Which means poor little Rover, for the first minutes is spining in a circle going from girl to boy to girl to boy. He gets so dizzy, he vomits and passes out.